AJ on a Daikin Altherma HT: Malfunction of capacity setting
Short answer
On a Daikin heat pump, AJ means: Malfunction of capacity setting. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Malfunction of capacity setting.
Part involved: DHW heat-up.
When it shows: Operation and: 1. When the capacity code is not contained in the PC board’s memory, and the capacity setting adapter is not connected. 2. When a capacity that doesn’t exist for that unit is set.
Likely causes:
- You have forgotten to install the capacity setting adapter; Defect of indoor unit PC board (A4P).
Is it safe?
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
What you can check
- Read the full error code shown on the user interface and note it, with the date/time, for your installer.
- Check the domestic hot water / programme schedule and settings on the user interface (Daikin notes AH-00 can be caused by large hot-water draws or stopping the tank during disinfection).
What the engineer will check
- Follow the service-manual troubleshooting flow chart for this code: check you have forgotten to install the capacity setting adapter; check defect of indoor unit PC board (A4P).
Who to ask for: Heat pump installer / heating engineer.
Certification: G3 unvented hot water competence for cylinder safety components.
What you might notice
Sources
- Service Manual Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 151 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026